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Surface- and Tip-Enhanced Raman Spectroscopy as OperandoProbes for Monitoring and Understanding HeterogeneousCatalysis
Clare E. Harvey • Bert M. Weckhuysen
Received: 20 October 2014 / Accepted: 4 November 2014 / Published online: 16 November 2014
� The Author(s) 2014. This article is published with open access at Springerlink.com
Abstract Surface-enhanced Raman spectroscopy (SERS)
and tip-enhanced Raman spectroscopy (TERS) were until
recently limited in their applicability to the majority of
heterogeneous catalytic reactions. Recent developments
begin to resolve the conflicting experimental requirements
for SERS and TERS on the one hand, and heterogeneous
catalysis on the other hand. This article discusses the
development and use of SERS and TERS to study heter-
ogeneous catalytic reactions, and the exciting possibilities
that may now be within reach thanks to the latest technical
developments. This will be illustrated with showcase
examples from photo- and electrocatalysis.
Keywords Heterogeneous catalysis � Raman � Scanning
probe microscopy � Colloidal synthesis � Nanostructure �Nanotechnology
1 Introduction
Surface-enhanced Raman scattering (SERS) is a phenom-
enon in which the Raman scattering intensity from mole-
cules close to the surface of certain finely divided metals is
amplified by several orders of magnitude [1]. As with
ordinary Raman spectroscopy, SERS spectra show the
molecular-vibration energies based on the frequency shift
between the incident and scattered light by matter. The
sensitivity enhancement of SERS in comparison to the
conventional Raman process has resulted in more
widespread applications, especially in surface chemistry
where vibrational spectroscopic data reveal how molecules
precisely interact with surfaces [1]. The analyte needs to be
adsorbed onto the surface of the SERS substrate or very
close to it (*10 nm), which can be challenging, however
this mechanism has the added value of quenching the
fluorescence from adsorbed molecules [2].
The unexpected enhancement of pyridine Raman signals
on Ag electrodes was first reported by Fleischmann et al. in
1974 [3]. Surprisingly strong and potential dependent
Raman signals from pyridine adsorbed on an electro-
chemically roughened Ag electrode have been observed.
Subsequent investigations by Albrecht and Creighton [4]
and Jeanmaire and Van Duyne [5] in 1977, led to the
discovery of SERS and its remarkable enhancement
effects. It was proposed by Van Duyne and co-workers that
the enhancement of the Raman scattering was due to an
increase in the electromagnetic field at the roughened
surface. Creighten et al. hypothesised that resonance
Raman scattering was responsible due to the creation of a
charge-transfer absorption band between the adsorbate and
the surface. These two ideas, now known as the electro-
magnetic and chemical enhancement mechanisms, have
dominated the mechanistic debate in the SERS community
for the past 30 years, and most probably will continue to do
so.
The recent revival of interest in SERS as an interesting
and valuable approach to probe heterogeneous catalytic
reactions has incited a flurry of publications on the subject.
Possibly the most interesting reports now deal with the
developments leading to the use chemically inert SERS
probes [6, 7] as well as to methods for anchoring SERS-
active nanoparticles on the external or internal surface of
catalyst supports and molecular sieves [8, 9]. They repre-
sent exciting new steps on the road towards the wide-
C. E. Harvey � B. M. Weckhuysen (&)
Inorganic Chemistry and Catalysis group, Debye Institute for
Nanomaterials Science, Utrecht University, Universiteitsweg 99,
3584 CG Utrecht, The Netherlands
e-mail: b.m.weckhuysen@uu.nl
123
Catal Lett (2015) 145:40–57
DOI 10.1007/s10562-014-1420-4
ranging use of SERS as an operando probe for monitoring
and understanding heterogeneous catalysis.
In this article we discuss the use, recent developments
and potential of SERS and TERS in the field of heteroge-
neous catalysis. Special attention will be focused on some
historic developments, recent applications for e.g. reaction
monitoring and the technical requirements to be further
explored to make this methodology a real asset in the ever-
expanding toolbox of in situ spectroscopic methods for
heterogeneous catalysis research. The goal of this paper is,
however, not to give a comprehensive overview of the
entire fast-growing research field. Within this context, we
refer the reader to some older and more recent review
papers on this topic to put the described developments in
the proper context [10–12].
2 A Brief History to SERS of Heterogeneous Catalysis
2.1 1981–1986: First Applications of SERS in the Field
of Heterogeneous Catalysis
The first steps towards the use of SERS in the field of
heterogeneous catalysis were taken in the 1980s with the
study of adsorbates at catalytically relevant interfaces by
the Dorain group at Yale University. Initial work was done
on Ag powders with a rough surface on the nanoscale, on
which surface species such as SO32- and SO4
2- were
observed and identified [13–16]. These early forays into
kinetic studies utilising the SERS methodology were ten-
tative in their conclusions, as it was already noted early on
that the intensities of SERS peaks are dependent on the
metal surface roughness as well as on the adsorbate con-
centration. The SERS-active phase on the Ag powder was
formed in situ in the case of the reactions of NO2/N2O4
with Ag. SEM measurements were performed to confirm
the theory that Ag microstructures were formed on the
surface of the Ag powder. It was confirmed that the for-
mation of these microstructures was due to the surface
layer of Ag2O reacting with the initial pulse of the gas to
form AgNO3 ? Ag ? NO. The Ag atoms formed then
migrated freely due to the thermal energy released by the
reaction, forming Ag microstructures. These first series of
studies demonstrated the potential of SERS for studying
heterogeneous catalysis, as well as highlighting the chal-
lenges of separating the catalytic reaction from the SERS
analysis.
2.2 1987–1993: Introduction of SERS in the Field
of Electro-catalysis
Following the first reported successes of SERS for heter-
ogeneous catalysis research it could have been expected
that the field of ultra-high vacuum (UHV) surface science
would take on the challenge of SERS, but this was not
really the case. The early recognition that the SERS effect
was limited to roughened surfaces of ‘coinage’ metals most
probably dampened the interest of the UHV surface-sci-
ence community. The concurrent emergence of electron
energy loss spectroscopy (EELS) and infrared-reflection
absorption spectroscopy (IRAS) for UHV-based systems
most probably encouraged this indifference, as unlike
SERS these vibrational techniques were applicable to the
ordered mono-crystalline metal surfaces that were at least
in the early days the primary focus of the UHV community.
The markedly greater interest shown by the electrochemi-
cal community in SERS was probably due in part to the
near-exclusive usage (at that time) of polycrystalline metal
electrodes, along with the straightforward preparation of
SERS-active surfaces by means of controlled-potential
oxidation–reduction cycles.
It is important to mention here that the group of Michael
Weaver at Purdue University took a new approach to SERS
substrates in the late 1980s by coating Au electrodes with
thin layers (3–4 monolayer) of transition metals, such as
Ru, Rh and Pt [17]. These thin metal layers provided the
catalytic surface, whilst the Au electrode beneath enhanced
the Raman scattering.
The transition metal thin films were electrodeposited
onto an electrochemically roughened gold electrode. The
thickness of these transition metal layers can be controlled
by varying the potential across the electrode. By utilising
transition metals in this way, a much broader range of
catalytic reactions were now available for study. The
Weaver group investigated catalytic systems ranging from
CO and SO2 oxidation to methanol and formic acid oxi-
dation [18–21].
An early example of these studies is the electro-oxida-
tion of CO over Rh and Ru coated Au electrodes, followed
by the observation of SERS bands in the C–O stretching
region, which disappeared upon voltage increase at the
electrode [22]. Corresponding cyclic voltammetry mea-
surements show CO electro-oxidation occurring at the
same voltage at which the SERS bands disappear, con-
firming the reaction. However, in 1988 it still took 10 min
to collect these SERS spectra, preventing practical kinetic
measurements.
A significant discovery enabled through the use of
in situ SERS is exemplified in the case of methanol oxi-
dation, SERS spectra and cyclic voltammograms of which
are shown in (Fig. 1) [21]. It was thought that methanol
oxidation over Au was not active because the intermediates
were not formed on the Au surface, thus preventing reac-
tion. However, the SERS results clearly showed that the
required formate intermediates were indeed formed on the
Au surface. This demonstrated that the molecular
Surface- and Tip-Enhanced Raman Spectroscopy as Operando Probes 41
123
fragmentation of methanol, though necessary, was not in
itself sufficient, and that it was in fact the availability of
oxygen co-adsorbates that was the controlling factor in
methanol oxidation.
2.3 1992–1999: Moving Beyond the Field of Electro-
catalysis
An important limitation of the transition metal thin films
approach developed and used by Weaver and co-workers in
the late 1980s and early 1990s was the presence of residual
‘pinhole’ sites that exposed the underlying Au substrate.
However, this complication turned out to be much less of
an impediment to the utilization of the technique in gas-
phase environments, particularly at the elevated tempera-
tures of catalytic relevance, since Au displays near-negli-
gible adsorptive properties under these conditions. It was
found that the Pt-, Rh- and Ru- coated Au surfaces
exhibited excellent temporal stability when under ambient
pressure gas-phase environments, which allowed the
exploration of the utility of the SERS approach for probing
the adsorption of molecules at such catalytic relevant sur-
faces [23].
The advent of CCD technology enabled spectra to be
obtained at significantly faster rates (1–2 s). This technical
advance also lead to the improved sensitivity in Raman
measurements, as demonstrated by McCreery and co-
workers detecting adsorbates at the carbon-aqueous inter-
faces even in the absence of SERS, [24] and the Tian group
obtaining Raman spectra for a diverse range of adsorbates
at the transition metal-solution interface in the presence of
only mild surface enhancements, more specifically
*30–100 [25, 26].
The Weaver research group built upon their experience
in the SERS field to move from electrochemical cells to
gas-flow reaction systems. The combination of in situ
SERS and in-line mass spectrometry analysis of the gas
flow enabled them to investigate a range of chemical
reactions. The changing intensities of Raman bands of
surface species and simultaneous analysis of the reaction
products enabled new physicochemical insight into the
reaction kinetics and related mechanisms over transition
metal surfaces.
The catalyst systems investigated include the in situ
SERS on the stability and reactivity of adsorbates,
including SO2, NO, CO, H2, O2 and methanol, up to tem-
peratures of 500 �C. A schematic of the set-up, along with
a selection of SERS spectra for a reduced Rh surface in
contact with NO are shown in Fig. 2. More specifically, the
initial work in the gas flow system established the
adsorption spectra of M–NO, M–Co and M–O, as well as
their stability at temperatures of up to 350 �C [28]. This
Fig. 1 Potential-dependent SER spectral sequence obtained on gold
during voltammogram at 10 mV s-1 for 0.5 mM methanol in 0.1 M
HClO4 from -0.3 to 1.5 V versus SCE and return (left). Intensity of
mCO band (arbitrary scale) versus electrode potential extracted from
SER spectra on gold in 0.1 M KOH with 0.1 M methanol, during
10 mV s-1 potential sweep from -0.8 to 0.6 V and return. The open
and filled circles refer to the forward and reverse sweeps, respectively
(right). Corresponding voltammograms obtained in presence (solid
trace) and absence (dashed trace) of 0.1 M methanol. Reproduced
with permission from Ref. [21]. Copyright American Chemical
Society, 1993
42 C. E. Harvey, B. M. Weckhuysen
123
was followed by work in which intermediate cyanide sur-
face species were spectroscopically observed at tempera-
tures above 185 �C during the oxidation of CO by NO over
Rh-coated Au surfaces [29]. The further use of on-line
mass spectrometry to detect the reaction products during
temperature ramps and gas-flow composition elucidated
even more information [30]. Using their knowledge of
these methods, the Weaver group went on to study the
surface species and products of the NO–H2 reaction on Rh,
the CO–NO reaction of Pt and Pd surfaces, as well as the
methanol oxidation on Rh surfaces [27, 31–36]. Also
studied were the metal oxides of the Pt group, their for-
mation as a function of temperature, and their reduction
kinetics and thermal oxidation [37, 38].
3 Heterogeneous Catalysis and SERS: Developments
in the Last Decade
Single-molecule SERS (SM-SERS) was first demonstrated
in 1997, garnering new interest in SERS from a much
wider audience. Advances in nanotechnology and synthetic
methods have also assisted in this new wave of research,
leading to several significant advances in the field.
3.1 Revival of SERS in the Field of Electro-Catalysis
Following a 5-year hiatus, a revival of interest in the use of
electrochemical SERS around the turn of the millennium
led to an expansion of the scientific community involved in
the study of SERS of heterogeneous catalysis.
It is worth noting that many SERS studies have been
undertaken on surface chemistry and molecules adsorbed at
metal electrodes. These are often closely related to the
catalytic reactions discussed in this paper, however to
discuss these fully is beyond the scope of this work. Some
noteworthy examples include the seminal SERS and TERS
work of Ertl and Pettinger, an example of which is show-
cased in Fig. 3 [39–42].
The Weaver group returned to SERS studies of electro-
catalysis after several years of focussing on utilising SERS
Fig. 2 (left) Schematic of the SERS gas-flow set-up; and (right)
time-resolved SER spectra for a reduced Rh surface exposed to NO.
The surface was first treated with CO at room temperature, evacuated,
and then held in 80 ml/min Ar. NO at 10 ml/min was then added to
the gas feed. Spectra shown are for times of a 0, b 30, c 60, d 120, and
e 240 s after the NO flow was initiated. Reproduced with permission
from Ref. [27]. Copyright Elsevier, 1996
Fig. 3 (left) SERS and TERS for Brilliant Cresyl Blue (BCB) on a
mildly activated Au film; and (right) Resonance Raman Scattering
(RRS) and TERS for BCB at a smooth Au film with a thickness of
12 nm. Reproduced with permission from Ref. [42]. Copyright
Wiley–VCH, 2002
Surface- and Tip-Enhanced Raman Spectroscopy as Operando Probes 43
123
for gas-phase reactions, publishing a couple of new electro-
catalytic SERS articles in the midst of more fundamental
electrochemical work [43–45]. The death of Weaver in
2002 clearly left a vacuum in the fields of electrochemistry
and SERS that was later on filled by several new research
groups. In this respect, it is worthwhile to mention that a
special edition of The Journal of Electroanalytical Chem-
istry was dedicated to the memory of Michael, to which
Ertl and co-workers, as well as Koper et al., contributed
papers on SERS in the field of electro-catalysis [46, 47].
In 1989 Zhong-Qun Tian was appointed at Xiamen State
University, and focused his research on electrochemical
SERS and its application in the field of heterogeneous
catalysis. This research group revisited and made important
improvements upon the older electro-catalytic work of the
Weaver group [48–52], progressively making their own
way in the field first with a new electrochemical cell with
heating capabilities, [53] and then later on working by
working with Ag electrodes. [54, 55] In 2006 the method
for Pt-shell Au-core nanoparticles was developed as an
alternative to transition metal-coated electrodes. [56, 57]
This important advance allowed researchers to utilise the
high SERS intensity formed at hotspots between nanopar-
ticles without exposing the Au nanoparticle surface to the
chemical reaction, essentially decoupling the catalysis and
sensing/enhancing properties of the metal surface. This is
illustrated in Fig. 4 for CO adsorption on Au nanoparticles
surrounded by a Pt shell of various thickness.
A new method for coating SERS-active electrodes with
a monolayer of small Pt or Pd nanoparticles was developed
in 2005 by the Perez group in Alicante [58–61].The
nanoparticles coating of the electrode are small enough (i.e,
4 nm) that the SERS enhancement is maintained, whilst the
use of a layer of nanoparticles rather than an ultrathin layer
of Pt metal over the electrode gives an improvement in
stability. A combined SERS and AFM study was under-
taken by Scheijen et al. in 2007, looking at the influence of
the Pt nanoparticles deposition procedure over Au
electrodes on the electro-oxidation of small organic mol-
ecules. These authors concluded that the method of nano-
particles deposition has a strong influence on the size and
homogeneity of the nanoparticles, and the resulting cata-
lytic and SERS activity [62].
Studies of the electrochemical reduction of keto-esters
over Pt surfaces have been undertaken by Attard et al. at
Cardiff University, using the previously mentioned Tian
method of coating Au nanoparticles with Pt. Mechanistic
insight into these chemical reactions show the potential of
the SERS methodology for measuring increasingly com-
plex catalytic systems [63, 64]. The electrochemical SERS
work of Koper and co-workers at Leiden University also
utilizes a variety of bare and transition metals-coated
electrodes, in combination with which SERS is used to gain
a deeper understanding of the systems involved in solar
fuels production [47, 62, 65, 66]. Along the same line of
research, recent publications of the Bell group at UC
Berkeley report on the use of cobalt oxide- and nickel
oxide-coated Au electrodes to follow the electrochemical
evolution of O2 [67, 68].
3.2 Applications of Anchored SERS Nanoparticles
Moving towards the implementation of the SERS meth-
odology in more industrially relevant catalytic systems, the
SERS substrates are required to be physically robust as
well as sufficiently Raman signal enhancing. Various
methodologies for anchoring SERS nanoparticles to sup-
port materials have been reported, which look promising
for future use in heterogeneous catalysis. Some highlights
on these approaches are discussed below.
In 2004 a core–shell Au–Pt nanoparticle film was fab-
ricated by a self-assembly method on a silicon wafer, and
its application as a catalyst and as SERS substrates was
investigated [69]. The nanostructured film exhibited high
catalytic activity and SERS, demonstrating its potential use
in heterogeneous catalysis and as a SERS substrate. In
Fig. 4 (right) SER spectra of
CO adsorbed on 55 nm sized Pt-
coated Au nanoparticles with
different Pt shell thicknesses in
CO-saturated 0.1 M HClO4 at
0.0 V; and (left) 3D-FDTD
simulated electric field
distribution for a 1.5 nm Pt
layer surrounding a 55 nm Au
nanoparticle. Reproduced with
permission from Ref. [56].
Copyright American Chemical
Society, 2006
44 C. E. Harvey, B. M. Weckhuysen
123
addition, it should be possible to tune the surface properties
of the film by controlling the size, composition and surface
properties of core–shell nanoparticles. The self-assembly
method leads to a highly reproducible substrate, with great
potential for sensing applications in catalysis and other
fields.
Heck et al. reported that sub-monolayer coverage of Pd
on Au nanoparticles showed unexpected high activity for
the aqueous-phase hydrodechlorination of trichloroethene
[70]. This nano-shell SERS sensing approach was com-
bined with sub-monolayer Pd deposition to combine the
catalytic and SERS-active functionalities, enabling the
operando monitoring of catalytic activity. The nanoparti-
cles were immobilised on Si wafers and plasma-cleaned
prior to adsorption and reaction experiments in the aqueous
phase. UV–Vis and SERS spectra of the uncoated and Pd-
coated Au shells are shown in Fig. 5. Chemical reactions of
the adsorbate species were observed as they proceeded on
the catalyst surface with increasing reaction time, enabling
the detection and identification of reaction intermediates.
The authors express their hope that this showcase example
of nano-shell may further the SERS approach to gather new
mechanistic insights into other liquid-phase chemical
reactions.
Another promising approach makes use of mesoporous
silica. Mesoporous silicas are a well- known class of cat-
alyst supports that have large pore sizes and volumes as
well as well-ordered structures. In 2011, Silva et al. dem-
onstrated SERS enhancement from Au nanoparticles within
the mesoporous channel pores of SBA-15 [8]. An overview
of the ion-exchange synthesis method is shown in Fig. 6,
along with two examples of SERS spectra obtained using
the Au/SBA-15 material. Using 4-mercaptopyridine (4-
Mpy) as the SERS reporter molecule, SERS enhancement
factors as high as 105 could be obtained. The high SERS
enhancement given by the Au/SBA-15 as compared to any
other powered SERS substrates appears to be the result of
the formation of SERS ‘‘hot spots’’ due to the side-by-side
alignment of Au nanoparticles within the cylindrical
channel pores of the mesoporous silica host. Because of
their powdered forms, longevity and high SERS enhance-
ment factors, the Au/SBA-15 material can be expected to
serve as a simple substrate for SERS-based analysis of
various analytes, including reactants and reaction products
during heterogeneous catalysis. The potential for the
material has already been recognised, with a new, simpli-
fied method of preparing Au nanoparticles inside the SBA-
15 channels recently published [71].
Fig. 5 a UV–Vis spectra of Au nanoparticles and Pd/Au nanopar-
ticles with 10 % Pd coverage; b calculated spectra of Au and Pd/Au
nanoparticles; c Raman spectra of pMA chemisorbed on Au
nanoparticles and Pd/Au nanoparticles with 10 % Pd coverage with
the inset illustrating the metal-S stretching region; and d Raman
spectra of 1,1-DCE in H2O (50.9 lM) on Au nanoparticles and Pd/Au
nanoparticles with 10 % Pd coverage. Reproduced with permission
from Ref. [70]. Copyright American Chemical Society 2008
Surface- and Tip-Enhanced Raman Spectroscopy as Operando Probes 45
123
A recent promising nanomaterial is graphene, and has
been recently demonstrated as a support to disperse and
stabilize various metal and metal oxide nanoparticles.
Huang et al. describe the formation of Au nanoparticle-
graphene oxide (Au–GO) and Au nanoparticle-reduced GO
(Au-rGO) composites [72]. These composite materials
were used to demonstrate SERS of the molecule p-ami-
nothiophenole (pATP) as well as of the catalytic reduction
of o-nitro-aniline. The impressive results for both appli-
cations due to the electronic interaction of the graphene
with the Au nanoparticles, show great promise for the
possible combination of SERS and catalysis over graphene
substrates, and have already inspired several further
investigations into the use of graphene oxide in combina-
tion with SERS [73–75].
In 2012, different shapes of Pt nanoparticles were used to
enable the monitoring of a surface reaction on different
surface sites. Spherical and octahedral–tetrahedral Pt nano-
particles were used with the (111) surface orientation of the
octahedral–tetrahedral nanoparticles facilitating the reduc-
tion of acetaldehyde oxime into ethylamine. This reduction
process was not observed over spherical nanoparticles,
demonstrating the combination of SERS-active metals and
the use of preferentially ordered nanoparticles allows SERS
studies of structure-sensitive surface reactions. [76]
3.3 Monitoring Chemical Reactions Directly
in Colloidal Suspension
The Schlucker group of the University of Osnabruck
demonstrated in 2011 the successful synthesis of so-called
bifunctional Au/Pt/Au nanoraspberries [77]. These peculiar
nanoparticles successfully integrated the SERS activity of
Au nanoparticles with the catalytic activity of the Pt sur-
face, whilst enabling the monitoring of chemical reactions
directly in colloidal suspension. This is illustrated in Fig. 7.
They also presented a multivariate approach for the quan-
titative analysis of in situ SERS spectra that allows to
determine the chemical identity of the involved molecular
species and the quantification of their relative contribu-
tions, which is a requirement for establishing reaction
mechanisms and testing the related kinetic models. Finally,
Schlucker et al. noted that the presented approach of using
bifunctional Au/Pt/Au nanoraspberries for SERS monitor-
ing of Pt-catalyzed reactions is currently limited to
molecular species containing a surface-seeking group in
order to experience the necessary SERS enhancement.
Fig. 6 Schematic of the synthesis of Au nanoparticles within the
pores of SBA-15 (Au/SBA-15) (top) by sequential redox and galvanic
reactions through an intermediate product, Ag/SBA-15; (bottom left)
(a) Raman spectrum of 1 9 10-2 M 4-Mpy that was drop-casted and
dried on a CH3-capped SBA-15 sample, and (b) SERS spectrum of
1 9 10-5 M 4-Mpy that was drop-casted and dried over Au/SBA-15,
which was prepared in the presence of CTAB; (bottom right)
(a) Raman spectrum of 1 9 10-2 M 4-Mpy dried over SBA-15, and
(b) SERS spectrum of 1 9 10-5 M 4-Mpy dried over Au/SBA-15.
Reproduced with permission from [8]. Copyright American Chemical
Society, 2011
Fig. 7 SER spectra recorded during the Pt-catalyzed hydride reduc-
tion of an aromatic nitro compound using different amounts of the
reducing reagent NaBH4. From bottom to top: increasing volumes of
10 mM NaBH4 solution were added to 4-NTP-functionalized Au/Pt/
Au core/shell nanoraspberries. Reproduced with permission from Ref.
[77]. Copyright American Chemical Society 2011
46 C. E. Harvey, B. M. Weckhuysen
123
Joseph et al. have also recently used the idea of small
catalytic nanoparticles supported on large SERS nanopar-
ticles, following the catalytic reduction of pNTP with
NaBH4 over Pd nanoparticles of 2 nm in size supported on
40 nm-sized Au nanoparticles. [78]
More recent work by the Schlucker group has built upon
the principle of small catalytically active nanoparticles
supported on larger, SERS active nanoparticles [79]. The
80 nm Au core is encapsulated within a thin silica shell of
*1.5 nm in size, then thiol groups are used to attach 5 nm
Au nanoparticles to the surface. These bimetallic particles
are used to monitor the Au-catalysed reduction of pNTP in
colloidal suspension, whilst the inert silica shell protects
the Au metal core surface from direct contact with the
chemical species and prevents unwanted photocatalytic
side reactions.
Other Au and Ag nanostructures have been synthesised
and tested for both SERS and catalytic activity, though not
in combination. One such example is the three-dimensional
dendritic Au nanostructures developed by Huang et al.
[80]. The high surface-to-volume ratio of the dendritic
structures led to high catalytic efficiency, and the structure
provided multiple sharp corners and edges for SERS hot-
spots. However, the adaption of such substrates for use in
catalytic conditions could be complicated, and is clearly a
challenge for future research.
3.4 SERS and Plasmon-Driven Catalysis
For over a decade, pATP has been regarded as a proto-
typical probe for assessing the nature of SERS measure-
ments [81]. As pATP interacts strongly with Ag and Au,
gives strong SERS signals, and has been regarded as a
model compound for probing the chemical enhancement of
SERS due to its potential and wavelength dependent
spectra [82]. However, difficulties in reproducing the SERS
spectra of pATP alone led to further investigations, through
which two different explanations were derived. Sun and co-
workers first hypothesised that the catalytic conversion of
pATP to dimercaptoazobenzene (DMAB) accounted for
the unexplained SERS spectra in 2009 [83]. This conclu-
sion was supported by further theoretical and experimental
work by Sun and co-workers, who then also concluded that
DMAB could be produced by 4-nitrobenzenethiol (4-NBT)
[81, 84, 85]. It has been found that these plasmon-driven
reactions are strongly dependent on substrate, wavelength
and time. More significantly, these instances of plasmon-
driven catalysis have shown that SERS can be an invasive
technique under certain conditions, and that the species
measured may not be the original surface species [86]. In
contrast, the work of Kim et al. supports the hypothesis that
the SERS bands under debate are in fact a result of the
chemical enhancement mechanism [87–89].Within this
context it is important to refer to a recent study of Kang
et al. [90] of the Harbin Institute of Technology, who have
monitored the plasmon-driven conversion of pNTP into
DMAB using a single Ag particle of 2 lm in size and
having a roughened surface. This detailed SERS study
provided laser wavelength- and power-dependent conver-
sion rates for the reduction of pNTP into DMAB.
As a last example to illustrate the concept of plasmon-
driven catalysis we highlight recent research work of Tang
and co-workers of the Chinese Academy of Sciences in
Hefei. This group has developed SERS-active Ag particles
consisting of hierarchical peony-like microflowers with a
highly roughened surface, as illustrated in Fig. 8a. By
making use of these constructs in combination with the
dimerization reaction of para-nitrothiopenol (pNTP) into
DMAB is was possible to determine the detailed kinetics of
Fig. 8 a Schematic of the plasmon-driven surface catalysed reaction
of pNTP dimerizing into DMAB monitored by a single particle SERS
substrate, more specifically consisting of hierarchical peony-like Ag
microflowers. b,c SERS spectra of pNTP and DMAB. d Color-coded
intensity of spatiotemporal SERS mapping under continuous
632.8 nm laser excitation. Reproduced with permission from [91].
Copyright Royal Society of Chemistry, 2014
Surface- and Tip-Enhanced Raman Spectroscopy as Operando Probes 47
123
this process as a function of the different morphologies of
the microflowers synthesized. Fully developed nanostruc-
tures not only showed a larger SERS enhancement, they
also lead to an increase in the reaction rate of the dimer-
ization process [91].
4 Challenges Imposed on SERS for Performing
Operando Spectroscopy Measurements
The true aim for the SERS methodology in catalysis
research is of course to perform operando measurements,
leading to new fundamental insights into catalytic inter-
mediates and related transition states. In order to achieve
this ambitious goal, further advances must be made to
existing SERS probes, enabling the use of SERS under the
often extreme conditions encountered inside a catalytic
reactor; i.e., at high temperatures and pressures. The most
important challenges that are yet to be fully overcome are
chemical stability, thermal stability, spectral reproducibil-
ity and data analysis. These challenges will now be dis-
cussed in more detail below.
4.1 Chemical Stability
One of the primary considerations in the use of SERS for
label-free monitoring of catalytic reactions is the exposure
of a chemical reaction to an additional transition metal
surface in the form of SERS nanoparticles or a planar
SERS substrate.
For true operando measurements that are unlimited by
the catalyst surface being Au or Ag, SERS surfaces with a
high level of chemical stability must be developed, or
adapted from existing particles or surfaces. One solution
would be to use a SERS surface that could be coated with
e.g. Al2O3, however under true reaction conditions it would
be preferable to have inert particles that could be distrib-
uted within the catalyst material, being preferentially
within its shaped form (i.e. catalyst body), without causing
any transport limitations or disruption to the gas flow in the
catalyst bed.
Previous work in the field of spectroscopy has seen
attempts to modify the surface specificity of SERS nano-
particles by various methods. One of the oldest of these
approaches is the functionalisation of the nanoparticle
surface with alkane thiols, for example, to enable SERS
measurements of neutral molecules that would not usually
interact with the charged capping of a SERS active nano-
particle [11]. Another method to bring SERS and hetero-
geneous catalysis together is the use of an ultra-thin shell of
other transition metals, such as Pt and Pd [17]. These
systems effectively ‘borrow’ the SERS enhancement from
the Au core nanoparticles, whilst presenting a catalytically
active transition metal outer surface. Until recently, the
destabilisation of the nanoparticles upon the removal of the
stabilising coating layer has prevented these forms of
SERS substrates from application beyond electrochemical
systems, though the work of the Schlucker group has begun
to address this important topic [79].
Shell isolated nanoparticle-enhanced Raman spectros-
copy (SHINERS) is a powerful SERS method first pro-
posed in 2010 by the group of Tian et al [6]. It works under
the premise that the formation of an ultrathin silica shell
around a SERS nanoparticle will prevent the direct chem-
ical interaction of an analyte with the nanoparticle, whilst
being thin enough to still be within the range of the elec-
tromagnetic enhancement mechanism of SERS. The silica
shell also prevents the nanostructure of the nanoparticle
from degrading on an atomic level, therefore making the
SERS signal more stable. The sensitivity of this innovative
method to the silica shell thickness is illustrated in Fig. 9.
The challenge of the interference of a SERS substrate in
a particular system is also a hot topic within e.g. the bio-
analysis field, where it has been noted that the exposed
metal surface of metal nanoparticles can easily adsorb
interfering molecules in the biological environment, lead-
ing to variations of the original SERS signals as well as
possible biotoxicity [92]. Common solutions include the
use of a biomolecule surface-coating, such as bovine serum
albumine (BVA), non-toxic polymer coatings, liposome
coatings and silica coatings. Such approaches could also
inspire the field of heterogeneous catalysis.
4.2 Thermal Stability
Chemical reactions often run well over 300 �C, so it must
be possible to use SERS under such conditions. The ther-
mal stability of nanoparticles is a well-known topic in
heterogeneous catalysis, with the sintering of nanoparticles
at high temperatures being a major field of study. One
commonly used solution in heterogeneous catalysis is the
synthesis of active nanoparticles within a confined micro-
or nanoporous system, which restricts the mobility of the
nanoparticles under heating, and so prevents sintering. As
discussed above, Silvia et al. utilized a similar concept for
their SBA-15 crystals containing Au nanoparticles, though
they did not extend their study to look at thermal effects
[8]. An investigation into the heating effects on nanopar-
ticle films by Kho et al. confirmed that lateral particle
diffusion as a result of heating results in a loss of SERS
signal [93]. Other studies focusing on roughened metal
substrates have observed some effect of heating on the
SERS activity, though the changes are reversible upon
cooling [94–96].
As discussed previously, high-temperature SERS has
been achieved with the use of electrochemically-roughened
48 C. E. Harvey, B. M. Weckhuysen
123
surfaces as SERS substrates. These transition metal sur-
faces demonstrate a much higher degree of thermal sta-
bility than the metal nanoparticles more frequently
employed in recent years, though they do not have the
extremely high SERS enhancement factors often seen with
metal nanoparticles.
Fig. 9 a Potential-dependent SHINERS spectra of hydrogen
adsorbed on a Pt(111) surface. Curve I, at -1.2 V; curve II, -
1.6 V; curve III, -1.9 V; curve IV, without Au/SiO2 nanoparticles;
curve V, with the thicker shell nanoparticles at -1.9 V; b potential-
dependent SHINERS spectra of hydrogen adsorbed on a Pt(111)
surface using Au/Al2O3 nanoparticles. Curve VI, -1.2 V; curve VII,
-1.6 V; curve VIII, -1.9 V; (c) SHINERS spectra on Si(111) wafer
treated with 98 % sulphuric acid (curve IX), 30 % HF solution (curve
X) and O2 plasma (curve XI). Reproduced with permission from Ref.
[6]. Copyright Nature Publishing Group 2010
Fig. 10 Surface plasmon
resonance (SPR) spectroscopy
of bare Ag nanoparticles (a) and
Ag nanoparticles coated with
1.0 nm atomic layer deposition
(ALD) Al2O3 (b) heated at
500 �C over time. (c) Plot of the
SPR shift of Ag nanoparticles
coated with 0.2 nm (grey
triangle), 0.4 nm (filled square),
and 1.0 nm (filled diamond)
ALD Al2O3 heated at 500 �C
over time. All samples were
heated under 1 Torr N2.
Reproduced with permission
from Ref. [7]. Copyright
American Chemical Society,
2007
Surface- and Tip-Enhanced Raman Spectroscopy as Operando Probes 49
123
The temperature dependence of SERS with metal
nanoparticles, while significantly influenced by sintering, is
also highly dependent on the effect of heating on the sur-
face plasmon resonance (SPR) of the metal nanoparticles,
and its response under heating. Several research groups
have found that the SPR bands for SERS substrates can
red-shift upon heating, with the effect being more dramatic
upon nanoparticles that planar substrates [97, 98]. Research
work in the Van Duyne group at Northwestern University
discovered that using atomic layer deposition (ALD) to
apply a thin alumina coating over the SERS substrate
significantly increases their thermal stability, the impres-
sive results of which are illustrated in Fig. 10 [7]. Naturally
a thicker coating leads to enhanced thermal stability,
though coatings of as little as 0.2 nm alumina led to a
substantial increase in stability. It is important to mention
here that SHINERS have the potential to extend this notion
of thermal stability to SERS nanoparticles [6].
4.3 Spectral Reproducibility and Interpretation
The lack of measurement reproducibility is a major limi-
tation of SERS measurements that must be taken into
consideration in all measurements, and is in no small way
also related to the surface chemistry and related stability of
SERS substrates. A lack of substrate generality is intrinsic
to the SERS mechanism, as one of the major SERS
mechanisms—the chemical enhancement mechanism—
works via a charge transfer mechanism. Therefore, if an
analyte is not chemically compatible with the surface
charge of the SERS substrate, signal enhancement is
unlikely to occur.
SERS substrates themselves must also be reproducible
synthetically for the results to be truly reproducible.
Highly-ordered planar substrates are known to give the
most regular SERS spectra, however they sacrifice the high
spectral intensity seen when SERS nanoparticles form so-
called ‘hot-spots’. The use of multivariate data analysis of
spectral variance in SERS measurements using colloids is
one potential method to untangle some of the variance seen
in SERS measurements. The method takes into account the
variation of SERS between different batches of colloids, as
the synthesis method may not have been reproduced
always exactly [99]. This observation also implies that
results may differ from one laboratory to another, and even
within the same laboratory variations in time may occur as
not always the same persons are in charge of synthesizing
the SERS substrates or SERS-active nanoparticles.
Combination of techniques, already employed fre-
quently in heterogeneous catalysis research, may shed
further light on the interpretation of SERS measurements.
For example, an integrated AFM—Raman instrument is
able to directly correlate single nanoparticles or clusters of
nanoparticles to spectra, and so give further insight into the
chemistry occurring on SERS substrates. The integration of
AFM with Raman spectroscopy has been recently illus-
trated by Harvey et al. for the photo-degradation of rho-
damine-6G in combination with differently sized and
shaped Ag nanoparticles in the presence and absence of air
and at elevated temperatures by making use of an in situ
reaction chamber [100].
5 Showcase SERS and TERS Studies from Our
Laboratory
Recent SERS and TERS work from our group has focused
on two photo-catalytic/plasmon-driven catalytic reactions
involving two thiophenols, namely pNTP and pATP. Both
photo-reactive molecules have been largely studied in the
literature, as discussed before, as their reactivity can be
tuned by altering the laser excitation wavelength as well as
power. In other words, the reduction processes of pNTP
and pATP can be regarded as ideal model reactions for the
(further) development of the SERS and TERS spectro-
scopic toolkit for heterogeneous catalysis research.
5.1 SERS of the Photo-catalytic Reduction of pNTP
in a Self-assembled Monolayer
A large discussion point in the literature is the interpreta-
tion of the reaction product SERS spectrum during the
photo-catalytic reduction of pNTP. There exist mainly two
proposals: p-aminothiophenol (pATP) and p,p0-dimercap-
toazobisbenzene (DMAB). The challenge in the identifi-
cation of the reaction product is at least twofold: (1) an
extremely low number of molecules that are converted
during the photo-catalytic reduction reaction, and (2) the
few molecules present or formed are chemically bound to
the surface, and therefore non-accessible by other charac-
terization methods with molecular fingerprinting
capabilities.
In an attempt to resolve this issue, Van Schrojenstein
Lantman and co-workers have developed a two-dimen-
sional approach making use of a self-assembled monolayer
(SAM) of pNTP with Ag island films as SERS substrate to
determine the reaction kinetics upon laser irradiation
making use of SERS [101]. This approach is outlined in
Fig. 11. By following the characteristic Raman peaks of
pNTP and the reaction product it was possible to reveal in
two distinctly different ways that this photo-catalytic
reduction process involves a dimerization reaction, point-
ing towards the assignment of the reaction product as
DMAB.
The first approach involved the determination of the
reaction kinetics, as illustrated in Fig. 12. The two
50 C. E. Harvey, B. M. Weckhuysen
123
characteristic Raman peaks for tracking the reaction are
1,335 cm-1 (reactant pNTP, asymmetric NO2-stretch) [22]
and 1,440 cm-1 (product, assigned to pATP as a b2-mode
[23], or to DMAB as a N=N stretch) [13a]. Figure 12
shows that it is possible to obtain an idea of the surface
concentration of the reactant and reaction product by
tracking the peak areas of both species as a function of time
upon green laser light irradiation. It was found that the
concentration of pNTP decays in time, while synchro-
nously the reaction product of the photo-catalytic process is
formed. It was postulated that the reaction mechanism [24]
has a single rate-determining step and consequently the
reaction was expected to be a one-step process. In this
manner, the reaction order could be determined for the
rate-limiting step and turned out to be two.
The second approach providing evidence for a dimer-
ization reaction of pNTP was obtained through incorpora-
tion of an inert thiol into the SAMs of pNTP, as illustrated
in Fig. 11b. Thiophenol was here used as the two-dimen-
sional equivalent of a solvent. It was found that the reaction
Fig. 11 a SERS approach on a self-assembled monolayer (SAM) on
Ag island films as SERS substrate in which only the photo-catalytic
reaction at the surface is studied as a function of the irradiation with a
532 nm laser and (b) The photo-reduction of p-nitrothiophenol to
either p-aminothiophenol or p,p0-dimercaptoazobisbenzene. For dilu-
tion experiments, thiophenol has been used as the 2D-equivalent of a
solvent molecule. Reproduced with permission from Ref. [101].
Copyright Wiley–VCH 2014
Fig. 12 a Time-dependent
SERS measurements at 532 nm
laser irradiation from a pNTP-
coated Ag island film, each
horizontal line represents a
(colour-coded) spectrum. b The
first (top) and last (bottom)
spectrum of (a). The coloured
bands depict peak integration
area for quantification of pNTP
(1293–1373 cm-1 and product
(1,412–1,473 cm-1) in time.
c Time traces for pNTP (red)
and product (blue) as taken from
peak areas shown in (b).
d Relative peak area as function
of time. Reproduced with
permission from Ref. [101].
Copyright Wiley–VCH 2014
Surface- and Tip-Enhanced Raman Spectroscopy as Operando Probes 51
123
rate was extremely slowed down by the dilution effect with
thiophenol, up to the point that no photo-reduction reaction
was observed under the standard reaction conditions
applied. As a result, the reduction process of pNTP should
be second-order.
5.2 SHINERS Studies of the Photo-catalytic Reduction
of pATP
Harvey and co-workers have recently assessed the perfor-
mance of SHINERS for investigating the photo-reactivity
of the anticipated reaction intermediate in the process of
the photo-catalytic reduction of pNTP, i.e., pATP [102]. It
is known that pATP, as pNTP, interacts strongly with
either Ag and Au, giving strong SERS signals. As men-
tioned before, analogous to pNTP also pATP is regarded a
model compound for probing SERS activity. However, it
was noted before that extra Raman bands form in the SERS
spectra with an excitation wavelength of 633 nm or below.
Difficulties in producing the SERS spectra of pure pATP
led to detailed investigations of this effect, from which two
potential explanations were derived. The first explanation
involves the catalytic conversion of pATP into DMAB,
which is then considered to be the molecule responsible for
the changes in the SERS spectra of pATP [102]. This line
of thinking is supported by further theoretical and experi-
mental work, as discussed in detail above and is regarded
as a plasmon-driven reaction, similar pNTP. Another
explanation has been put forward by Kim et al., suggesting
that the SERS bands under debate are simply a result of
charge transfer, and therefore a reversible chemical
enhancement [87–89].
In order to shed further insights in both hypotheses,
Harvey et al. have investigated the behaviour of pATP
under laser light irradiation in the presence of both bare
gold nanoparticles and Au-based SHINERS [102]. This
was done using a 785 nm laser excitation to record the
initial pATP spectra, and a 633 nm laser excitation to
trigger the spectral changes in the Raman spectra. The
methodology used is outlined in Fig. 13, while the obtained
results are summarized in Fig. 14. Theoretically, if a
charge transfer process through the Au nanoparticles is the
cause of the unexpected spectral bands at 1140, 1392 and
1431 cm-1, these Raman bands will be observed over
uncoated gold nanoparticles, but not over the Au-based
SHINERS. To obtain sufficient statistics maps of 2,000
spectra per sample were averaged using Matlab to give the
average spectra shown in Fig. 14.
The dominance of the bands at 1064 and 1574 cm-1 in
the Raman spectra in Fig. 14a and c are indicative of the
presence of pATP, as exposure to 785 nm laser light results
only in extremely slow conversion of the starting material
into DMAB. The spectra in Fig. 14b and d, however, show
clear Raman bands at 1140, 1392 and 1431 cm-1,
Fig. 13 (a) and (b) High-
resolution transmission electron
microscopy (HRTEM) images
of a SHINER, consisting of a
silica-coated Au nanoparticle;
and c Schematic of a self-
assembled monolayer (SAM) of
pATP on gold on a silicon
substrate. Nanoparticles, either
gold nanoparticles or gold-
based SHINERS, are deposited
on top of this pATP monolayer.
The sample is then irradiated
with a 785 nm laser excitation
first, followed by an irradiation
with a 633 nm laser excitation.
After Ref. [102]
52 C. E. Harvey, B. M. Weckhuysen
123
indicative of the conversion of pATP over both bare gold
nanoparticles and Au-based SHINERS. The reason that no
complete conversion is observed could be linked to an
equilibrium state, and/or also to the fact that only a small
fraction of the observed molecules contribute to this signal.
The observation of the spectral changes over both bare gold
nanoparticles and Au-based SHINERS can be seen as a
confirmation that there is a photo-catalytic reduction of
pATP into DMAB as charge transfer cannot take place
through the oxidic and isolating shell surrounding the Au
nanoparticles, as illustrated in Fig. 13b by the HRTEM of
the Au-based SHINERS. This observation provides further
evidence that the hypothesis of Kim et al. [87–89] is rather
unlikely to occur and therefore we conclude that the origin
of the spectral changes is due to the formation of DMAB as
the dimerization product of pNTP, as has also been sup-
ported by theoretical and experimental work by Sun et al.
[81, 84, 85].
5.3 TERS Studies of the Photo-catalytic Reduction
of pNTP
In a recent study Van Schrojenstein Lantman and co-
workers reported on the first stepwise ‘react-analyse’
application of time-resolved TERS [103]. This unique
TERS approach is schematically illustrated in Fig. 15.
Here, the photo-catalytic reduction process of pNTP to
DMAB has been monitored by employing two different
excitation wavelengths: i.e., a green laser (532 nm) to
induce the photo-reaction, while a red laser (633 nm) was
used to probe the transformation process during the
reduction reaction with Raman spectroscopy. By exploiting
this multi-probe TERS methodology, it became possible to
Fig. 14 Raman spectra of a
surface layer of pATP in contact
with bare gold nanoparticles
after exposure to (a) 785 nm
laser light; (b) 633 nm laser
light; Raman spectra of a
surface layer of pATP in contact
with gold-based SHINERS after
exposure to (c) 785 nm laser
light; and (d) 633 nm laser
light. After Ref. [102]
Fig. 15 React-analyse TERS for monitoring the photo-catalytic
reduction of pNTP to DMAB making use of a dual laser excitation
approach. TERS spectra are measured making use of 633 nm laser
excitation, while the chemical reaction is triggered by a 532 nm laser
when the Ag-coated AFM tip is in contact with the pNTP monolayer.
Redrawn with permission from Ref. [103]. Copyright Nature
Publishing Company 2012
Surface- and Tip-Enhanced Raman Spectroscopy as Operando Probes 53
123
follow the catalytic activity of a Ag-coated TERS-tip. By
making use of a SAM of pNTP on a Au nanoplate it was
ensured that no more than a monolayer of reactant is
studied. As only the utmost point of the TERS tip is in
contact with the reactant, one can consider this is a first of
its kind study of a single catalyst ‘particle’ in action.
The combination of green and red laser sources was
found to be crucial for controlled observation of the photo-
catalytic reaction of pNTP to DMAB at the catalytic site, as
shown in Fig. 16 [103]. Upon green laser excitation the
photo-catalytic reduction reaction can be temporarily
activated and the typical Raman bands of pNTP and
DMAB at around 1,335 and 1,440 cm-1 can be followed as
a function of time during this reduction process at the
nanoscale.
6 Future Perspectives
With significant advances being made rapidly throughout
the SERS community, the prospect of performing operando
SERS and TERS measurements of a catalytic reaction are
clearly on the horizon. The introduction and related ver-
satility of SHINERS in the spectroscopic toolkit is an
important step forward along the way to introducing the
SERS method as a core analysis technique for heteroge-
neous catalysis research.
Already in recent times a couple of preliminary studies
for utilizing tip-enhanced Raman spectroscopy (TERS) in
catalysis have demonstrated its potential [104, 105]. As
shown in one of the showcase examples above, Van
Schrojenstein Lantman and co-workers used TERS to fol-
low a photo-activated catalytic reaction on the nanoscale,
and research is ongoing towards single molecule TERS
sensitivity and of improving the spatial resolution of this
optical technique into the nanometer scale, as well as
expanding the measurement conditions towards more
realistic ones [103]. Such operando TERS approach is
schematically shown in Fig. 17a.
The advent of surface-enhanced spatially-offset Raman
spectroscopy (SESORS), thus far mainly applied to bio-
logical systems, also demonstrates great potential for
application in catalysis [106, 107]. Perhaps the most sig-
nificant challenge in the full applicability of SERS as a
standard spectroscopic tool for catalysis research is fur-
thering the work done thus far on the chemical generality
or SERS, developing either a single method or a whole
toolkit of SERS substrates that enable the detection of any
analyte, irrespective of charge or hydrophobicity/hydro-
philicity. The first steps have been taken along this
Fig. 16 a Time-dependent
TERS spectra at 633 nm
excitation (5 s integration time,
380 lW) shown before (top)
and after (below white band)
illumination; b two spectra from
a are shown: spectrum (i) is
taken at 90 s and spectrum (ii)
at 265 s. Spectrum (iii) is the
reference spectrum taken after
the time-dependent spectra.
Asterisks in (a) and (b) indicate
the location of the 950 cm-1
band of the SiO2–glass signal of
the glass substrate; c peak areas
as a function of time for the
pNTP band at 1,335 cm-1
(i) and for the band at
1,440 cm-1 (ii), belonging to
DMAB. The period of green
illumination between 100 and
130 s is indicated by the shaded
band. Reproduced with
permission from Ref. [103].
Copyright Nature Publishing
Company 2012
54 C. E. Harvey, B. M. Weckhuysen
123
promising path, with the initial development of transition
metals-coated substrates, and more recently the invention
of SHINERS, the first SERS substrate to truly separate
heterogeneous catalysis and spectroscopic analysis (reac-
tion monitoring). [19, 108] SHINERS could really become
an asset for monitoring the reactants, reaction products, as
well as minute amounts of reaction intermediates, in a
reactor vessel, as illustrated in Fig. 17b. The showcase
example of Heck and co-workers is a clear indication that
this indeed may become possible. Moreover, more com-
binations of SHINERS and reactors and (individual) cata-
lyst particles and catalyst bodies can be envisaged,
providing local ‘‘Raman antenna’’ for catalytic perfor-
mance, including activity, selectivity as well as stability.
Acknowledgments This work is supported by the Netherlands
Research School Combination-Catalysis (NRSC-C), Netherlands
Organization for Scientific Research (CW-NWO) in the frame of a
Spinoza and Top research Grant, and an European Research Council
(ERC) Advanced Grant (No. 321140). The authors acknowledge
Evelien van Schrojentstein en Arjan Mank for their scientific con-
tributions to some of the research described in this article. This article
is partially based on Chapter 2 of the PhD thesis of Clare Harvey.
Finally, C.E.H. and B.M.W. acknowledge Agnieszka Ruppert for
making Fig. 17.
Open Access This article is distributed under the terms of the
Creative Commons Attribution License which permits any use, dis-
tribution, and reproduction in any medium, provided the original
author(s) and the source are credited.
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